mips64: fixed problem with Iop's for mips64.

Fixed problem with some Iop's that was discovered while running vbit-test
on mips64. Fixed operations are: Iop_1Uto64, Iop_Left64 and Iop_CmpwNEZ64.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2757 8f6e269a-dfd6-0310-a8e1-e2731360e62c